Protective device and method for use with audio amplification equipment

ABSTRACT

The user of a microphone or musical instrument connected to a mains amplifier by means of coaxial cable is protected from electric shock due to faulty equipment by means of sensitive fuses inserted respectively in the core and sheath circuits of the cable. If, due to a failure of the equipment, the user comes into contact with a high voltage through either the core or sheath circuit, the appropriate fuse will burn out, cutting off the current and preventing serious injury.

United States Patent Reid et al.

PROTECTIVE DEVICE AND METHOD FOR USE WITH AUDIO AMPLIFICATION EQUIPMENT Inventors: John S. Reid, 10 Boston Ave.,

Benton Lodge, Newcastle upon Tyne; Christopher W. J. Hayes, 8 Hector St., Shiremoor, both of England Filed: June 29, 1973 Appl. No.: 375,091

US. Cl. 317/40 A; 339/183 Int. Cl. H02H 3/08 Field of Search 317/40 A; 337/187, 188, 337/198, 199, 216, 255, 256, 263, 264, 269,

References Cited UNITED STATES PATENTS McBerty et a1. 337/283 Dec. 16, 1975 1,892,567 12/1932 Craddock 317/40 A 2,424,405 7/1947 Lytgens 338/263 2,548,491 4/1951 Peek, Jr. 317/40 A 3,281,756 10/1966 OKeefe et al. 339/143 R X 3,689,865 9/1972 Plei'ini et al 339/143 R X Primary Examiner-Harold Broome Assistant Examiner-Patrick R. Salce Attorney, Agent, or Firm-Larson, Taylor and Hinds 57 ABSTRACT The user of a microphone or musical instrument connected to a mains amplifier by means of coaxial cable is protected from electric shock due to faulty equipment by means of sensitive fuses inserted respectively in the core and sheath circuits of the cable. If, due to a failure of the equipment, the user comes into contact with a high voltage through either the core or sheath circuit, the appropriate fuse will burn out, cutting off the current and preventing serious injury.

2 Claims, 3 Drawing Figures US. Patent Dec. 16, 1975 PR OTECTIVE DIEVfC E vAND MEriion For: USE WITH AUDIO AMPLIFIC-ATION EQUIPMENT I lead of which i's'connected to the chassis'of the amplifier. The signal to be amplified is fed to the amplifier from a microphone or, for example, a musicalinstrument by means of a coaxial cable of which the outer sheath is joined to-the chassis :of the amplifier and thus to earth. The useof earthed coaxial cable is necessaryin order to avoid what is generally described as hum, hum being occasioned by stray signals picked up if other types of cable are used for the purpose.

Recently a number of accidents have been caused, and death has resulted, by users of equipment such as microphones or musical instruments receiving a severe shock from faulty amplification equipment, for example when the earth lead has not been properly connected or when the amplifier has been used with a 2-core cable circuit through a lighting system or the like.

SUMlVIARY OF THE INVENTION An object of the present invention is to reduce the likelihood of, or danger of injury from, electrical shocks to the users of amplification equipment, and can be achieved by fitting a sensitive fuse in the sheath circuit of the coaxial cable between the amplifier and the microphone or other instrument, and also a similar sensitive fuse in the core circuit of the coaxial cable, both fuses located preferably at a position adjacent the amplifier. By sensitive fuse we mean a fuse which will be broken by a current less than that likely to produce more than superficial injury to a person.

Accordingly, the present invention provides in one aspect a method of protecting users of audio amplification equipment having an amplifier with a coaxial cable leading to a microphone or musical instrument, which consists in fitting a sensitive fuse, as hereinbefore defined in the circuit containing the sheath of the coaxial cable between the amplifier and the microphone or musical instrument, and another such fuse in the core circuit of the cable.

By the use of our invention, if the chassisof an amplifier becomes live, for example by becoming connected in error to the mains supply, a performer touching any conductor, such as part of a microphone or musical instrument which is connected to or in contact with the sheath of the coaxial cable will become subject to the same voltage; if at the same time the performer makes good contact with earth potential, either for example through his feet, or through touching any element on the stage which is well earthed, a current will flow through his body, but if this reaches any dangerous limits the fuse in the sheath circuit will be broken andcut off the current flow. A 10 mA fuse is thought to, be a suitable value for the purpose. Although the performermight initially experience an electric shock, the immediate cutting off of the current would be likely to prevent more than superficial injury.

The requirement for two fuses is brought about by the fact that, if there is only .one fuse (in the sheath circuit), a .leakage current is still able to flow through the-coaxial cable and thus to the performer even after the sheath circuit has been broken. The current finds its way to the performer via the core of the coaxial cable, and the impedancepresented by the moving coil instrument .or microphone, between the core of the coaxial cable and the metal parts of the instrumentor microphone. v

It has been found that the presenceof a fuse in the sheath circuit can decrease the screening properties of the sheath.-ln-such cases we connecta capacitor across the fuse in the sheath circuit. This offers a low.impedance circuit to. RF interference but must present a sufficientlyhigh impedance to alternating currents at mains frequency to reduce any current passing there through at mains voltage to a value which is not likely to endanger human life; we have'found that 1000 picofarad capacitor is satisfactory.

It is convenient for the fuses (and the capacitor if used) to be incorporated in a self contained device. This device should be provided with means for connecting the two fuses respectively into the sheath circuit and the core circuit of the coaxial cable. These means may incorporate a jack socket and a jack plug, so that the device can be plugged in between the coaxial cable and the amplifying equipment.

Accordingly, the present invention provides in another aspect a protective device having a jack socket, two fuse holders and a jack plug, one fuse holder being connected on one side to the tubular contact of the jack plug and on the other side to the respective terminals of the jack socket adapted to make contact with the tubular contact of another jack plug, and the second fuse holder being connected on one side to the tip contact of the jack plug and on the other side to the respective terminal of the jack socket adapted to make contact with the tip contact of another jack plug.

Preferably the device should have a protective outer casing to enclose the fuse or fuses.

An embodiment of such a device is shown in the accompanying drawing wherein:

FIG. 1 is a side view of the device;

FIG. 2 is a longitudinal section through part of the device; and

FIG. 3 is a transverse section along the line III-III of FIG. 2.

It is usual for a microphone or musical instrument (as represented by signal producing apparatus block 2) to be connected by coaxial cable (as represented by the reference numeral 6) to a jack plug (as represented by the reference numeral 8) which is plugged into a socket in the amplifier (as represented by amplifier block 4). The protective device 10 is adapted to receive the jack plug 8 and itself carries a further jack plug 12 which is for fitting into the amplifier.

The device 10 has a protective casing consisting of a cylindrical part 14 of plastics material fitted with end caps 16. In one of the latter is secured a jack socket 18. Located within the casing 14 is a thin plate 20 of insulating material which is almost as wide as the internal diameter of the casing, so that it lies in position without 3 much lateral movement. Mounted on each side of the plate 20 is a pair of sockets 22, 24 respectively, each pair holding a mA fuse 26. One socket 22 is electrically connected to the sheath of a short length of coaxial cable 28, the other end of which is connected to the tubular contact 30 of the jack plug 12; the other socket 22 is electrically connected to the terminal of the jack socket 18 which is destined to make contact with the tubular contact of the jack plug 8 (this being connected to the sheath of coaxial cable 6). The sockets 24 are respectively joined to the core of cable 28 and thus to the tip contact 32 of the jack plug 12 and to the terminal of the jack socket 18 which is destined to make contact with the tip contact of the jack plug 8. Connected across the sockets 22 and thus in parallel with one fuse 26 is a 1000 picofarad capacitor 34.

What is claimed is: l. A means for protecting a user of audio amplification equipment against electric shock, said equipment.

comprising an amplifier, apparatus producing a signal to be amplified, and coaxial cable means for electrically connecting said amplifier to said apparatus, the coaxial cable means having a core circuit and a sheath circuit, and the protecting means comprising fuses, located respectively in said core and said sheath cir- 4 cuits between said amplifier and said apparatus, for isolating the user of the apparatus from the amplifier so that any current passing from the amplifier through the user to ground must pass through said fuses said fuses being such as will be broken by a current less than likely to cause more than superficial injury to a person, both said fuses being mounted in a single separable unit having two ends, there being a jack socket at one end and a first jack plug at the other, said jack plug being rotationally symmetrical and having a tip contact connected through one of said fuses to a contact of the jack socket in contact with the tip contact of a second jack plug, similar to the first, inserted into said jack socket, and a cylindrical sleeve contact connected through the other of said fuses to a contact of the jack socket in contact with the sleeve contact of said second jack plug, said second jack plug being connected by said coaxial cable means to said apparatus, and said first jack plug being inserted into a matching jack socket in said amplifier.

2. A means as claimed in claim 1 wherein said fuses can be broken by a current of more than about 10 milliamps 

1. A means for protecting a user of audio amplification equipment against electric shock, said equipment comprising an amplifier, apparatus producing a signal to be amplified, and coaxial cable means for electrically connecting said amplifier to said apparatus, the coaxial cable means having a core circuit and a sheath circuit, and the protecting means comprising fuses, located respectively in said core and said sheath circuits between said amplifier and said apparatus, for isolating the user of the apparatus from the amplifier so that any current passing from the amplifier through the user to ground must pass through said fuses said fuses being such as will be broken by a current less than likely to cause more than superficial injury to a person, both said fuses being mounted in a single separable unit having two ends, there being a jack socket at one end and a first jack plug at the other, said jack plug being rotationally symmetrical and having a tip contact connected through one of said fuses to a contact of the jack socket in contact with the tip contact of a second jack plug, similar to the first, inserted into said jack socket, and a cylindrical sleeve contact connected through the other of said fuses to a contact of the jack socket in contact with the sleeve contact of said second jack plug, said second jack plug being connected by said coaxial cable means to said apparatus, and said first jack plug being inserted into a matching jack socket in said amplifier.
 2. A means as claimed in claim 1 wherein said fuses can be broken by a current of more than about 10 milliamps. 